A constant voltage generating device is a generator capable of outputting a voltage that is practically unchanged despite variations in the ambient temperature. This type of generator is called "band-gap" device when it uses the property that the voltage at the terminals of one or several semiconductor junctions depends on the temperature, this voltage being a function of the width of the prohibited band (band-gap) of the semiconductor(s) considered.
A band-gap voltage reference has many applications in the microelectronics domain, and particularly for integrated electronics. For example, the band-gap voltage reference may be used as a set voltage value generator for a microprocessor power supply voltage monitoring circuit. It may also be used as a reference voltage generator for an analog-to-digital converter.
FIG. 1 shows a typical and known electrical diagram for a band-gap type constant voltage generator. The device in FIG. 1 comprises mainly a current source 10 connected in series with a resistance 12 called the adjustment resistance, and between a positive power supply terminal 14 and a ground terminal 16. An output monitoring transistor 18, electrically connected (through its gate) to a leg of a current source 10, is connected in series with a resistance 20, called the load resistance, between the power supply terminals 14, 16. The constant voltage output by the device and denoted V.sub.GAP is available between the ground terminal 16 and an output node 22 located between the output transistor 18 and the load resistance 20. In other words, the voltage V.sub.GAP is available at the terminals of the load resistance 20.
The current source comprises a first leg 10a with a first transistor 24a called the mirror transistor, in series with a second transistor 26a of the npn bipolar type, and a resistance 28 called the emitter resistance. The emitter resistance connects the emitter of the bipolar transistor 26a to a node 30. Furthermore, node 30 is connected to the ground terminal 16 through the adjustment resistance 12.
The first leg 10a is also called the pilot leg of the current source. A second leg 10b is connected in parallel to the first leg 10a between the positive power supply terminal 14 and the node 30. It comprises a first transistor 24b called the mirror and a second transistor 26b of the npn bipolar type, in series.
One terminal of the mirror transistor 24b and the collector of the bipolar transistor 26b in the second leg, are connected to the gate of the output follower transistor 18 at a node 32. Furthermore, the bases of the bipolar transistors 26a and 26b in the first and second legs are connected to the output node 22. According to an important characteristic of the band-gap type device shown in FIG. 1, the bipolar transistors in the two legs of the current source have different emitter surface areas. Thus, there is a difference in the base-emitter voltage for these transistors.
In the circuit shown in FIG. 1, it is considered that the bipolar transistor 26a in the first leg has a larger emitter surface than the emitter surface area of the bipolar transistor 26b in the second leg. The difference in the voltage between the base-emitter voltages V.sub.BE a and V.sub.BE b in the bipolar transistors 26a and 26b in the first and second legs respectively, is denoted .delta.V.sub.BE =V.sub.BE a-V.sub.BE b. This voltage difference is transferred to the terminals of the emitter resistance 28 through which a current I.sub.a passes such that: ##EQU1## where R.sub.1 is the value of the emitter resistance 28.
As a first approximation, it is estimated that the current I.sub.a corresponding to the emitter current in the bipolar transistor 26a in the first leg is the same as its collector current. The current I.sub.a is thus the current in the first leg 10a in the current source.
The mirror transistors 24a and 24b form a current mirror used to copy the current I.sub.a passing through the first pilot leg 10a to the second leg 10b. Let the current in the second leg 10b, i.e. the emitter current in the bipolar transistor 26b, be denoted as I.sub.b, and it is then found that I.sub.a .apprxeq.I.sub.b.
A current approximately equal to I.sub.a +I.sub.b =2I.sub.a passes through the adjustment resistance 12. Thus the voltage V.sub.GAP may be expressed by V.sub.GAP =V.sub.BE b+2R.sub.2 I.sub.a, where V.sub.BE b is the base-emitter voltage of the bipolar transistor 26b in the second leg, and R.sub.2 is the value of the adjustment resistance 12.
Knowing that the voltage V.sub.BE b reduces linearly with temperature, and that the current I.sub.a increases linearly with temperature, an appropriate choice of the value R.sub.2 of the adjustment resistance 12 will hold the output voltage V.sub.GAP approximately constant and independent of the temperature.
For guidance, note that the expression of .delta.V.sub.BE as a function of the temperature T is such that: ##EQU2##
In this formula, K is Boltzman's constant, q is the electron charge and S.sub.a and S.sub.b are the surface areas of transistor emitters 26a and 26b in the first and second legs respectively. Thus the current I.sub.a increases linearly with the temperature.
For example, the value R.sub.2 of the adjustment resistance is chosen such that the voltage V.sub.GAP is on the order of 1.2 V when the power supply voltage between the supply terminals 14, 16 is on the order of 5 V.
When constant voltage generators as described above are made in series, and particularly in integrated manufacturing, it is found that the characteristics of the components may vary from one device to another. In particular, the bipolar npn transistors used may have different saturation currents. The spread of characteristics will affect the value of the output voltage V.sub.GAP obtained. Thus, it is impossible to obtain constant voltage generators with almost identical output voltages in a production run.
This phenomenon is illustrated below in an example in which it is assumed that the bipolar transistors in each of the first and second legs have saturation currents that can vary between a typical value denoted I.sub.styp and a maximum value I.sub.smax. The base-emitter voltage in the bipolar transistor 26b in the second leg may thus vary as a function of its characteristics. The maximum variation is denoted here as .DELTA.V.sub.BE and is such that: ##EQU3##
The value of .DELTA.V.sub.BE at ambient temperature is -7 mV for I.sub.styp =3.09.times.10.sup.-17 A and I.sub.smax =4.13.times.10.sup.-17 A. This spread of the values of V.sub.BE of the transistors directly influences the output voltage V.sub.GAP of the devices. This calculation is approximate in that it ignores transistor base currents and the Early effect of transistors. Note that the Early voltage, denoted V.sub.AF in the rest of this description, measures the current sensitivity of a transistor as a function of the voltage variations (transistor output impedance).
Furthermore, taking account of the base current, it is observed that the emitter current I.sub.E of bipolar transistors is different from the collector current. Denoting the collector currents in transistors 26a and 26b in the first and second legs 10a and 10b as I.sub.a and I.sub.b, and their emitter currents as I.sub.Ea and I.sub.Eb, the following relations can be written: ##EQU4## the terms V.sub.CEa and V.sub.CEb referring to the collector-emitter voltages of the bipolar transistors in the first and second legs respectively, while .beta. is the gain of these transistors.
Considering that typical values of V.sub.AF and .beta. are V.sub.AFtyp =106 and .beta..sub.typ =142 for the bipolar transistors used, and considering that the maximum values are V.sub.AFmax =77.8 and .beta..sub.max =185, and considering that V.sub.CEa =2.7 V, V.sub.CEb =1.4 V for a power supply voltage of 5 V, correction factors X.sub.a and X.sub.b for I.sub.Ea and I.sub.Eb are obtained such that X.sub.a =0.9928 and X.sub.b =0.9969.
These correction factors are defined by: ##EQU5##
In these formulas, the typ and max indices denote the typical and maximum values respectively.
Denoting the voltage measured at the terminals of the adjustment resistance 12 as V.sub.R2, and the voltage difference measured at the terminals of the adjustment resistance 12 as .DELTA.V.sub.R2, and using typical values and maximum values of the transistor parameters, the following values are obtained: EQU V.sub.R2 =(I.sub.Ea +I.sub.Eb)xR.sub.2 ##EQU6##
In this expression, when a value of 600 mV (the case for typical values) is used for V.sub.R2, the voltage at the terminals of the adjustment resistance, we obtain .DELTA.V.sub.R2 .apprxeq.+3 mV. The voltage difference .DELTA.V.sub.R2 at the terminals of the adjustment resistance 12, partly compensates for the voltage difference .DELTA.V.sub.BE appearing at the bipolar transistor 26b on the second leg.
Thus, considering the basic current and the Early effect of bipolar transistors, the spread of transistor characteristics leads to a variation in the output voltage .DELTA.V.sub.GAP such that .DELTA.V.sub.GAP =.DELTA.V.sub.BE +.DELTA.V.sub.R2 =-7+3=-4 mV. Note that the output voltage generally produced by a constant voltage generating device such as that described above is on the order of 1.2 V. In many applications this voltage is used as a reference voltage and a dispersion or spread of 4 mV would be unacceptable.